The invention relates to a continuous-time sigma delta radio frequency modulator. It also relates to an analogue-to-digital radio frequency converter comprising such a modulator as well as an electronic device, such as for example a software-defined radio receiver, comprising such a modulator and/or such a converter.
The field of the invention is the field of processing of radio frequency (RF) signals, and more particularly the field of analogue-to-digital conversion of radio frequency signals, and even more particularly in software-defined radio applications, and in cognitive and opportunistic radio applications.
There is currently an increasing need for RF analogue-to-digital converters having a frequency band, called frequency band of interest, of several tens of megahertz (MHz) centred around a frequency of the order of one gigahertz (GHz). This need is met for example by the development of new radiocommunications techniques based on software-defined radio.
Existing RF analogue-to-digital converters are intended to convert the entire band of frequencies received (from DC to several GHz). These converters have the drawback of having a very high power consumption (several watts) and as a result are not suitable for portable electronic devices.
Analogue-to-digital converters of the bandpass Sigma Delta (SD) type constitute a promising response to this need, since the latter are capable of converting a limited frequency band around a certain central frequency. In order to achieve central frequencies of several GHz, the bandpass filter of this type of modulator is most often produced using passive LC resonators. However, the performance of this type of converters is inadequate, in particular in terms of signal-to-noise ratio (SNR).
A method of increasing the SNR, and therefore the performance of a bandpass SD converter, consists of increasing the order of the bandpass LC filter used in the loop of the modulator of an RF SD converter.
The standard solution for producing high-order LC SD modulators is to couple several LC resonators using a Gm transconductor.
FIG. 1 shows an example of such a 4th-order SD modulator. The modulator 1 comprises a processing chain 2 and a current feedback loop 3, not shown in detail. The feedback loop could also be a voltage feedback loop. Processing chain 2 receives an analogue current X(s) at an input 21 and delivers a digital signal Y(z) at an output 22. Processing chain 2 comprises, connected in series, a first LC resonator 23, a Gm transconductor 24, a second LC resonator 25, and a threshold comparator 26 working at a sampling frequency fs. The first LC resonator 23 is produced by connecting in parallel a capacitor C1 and an inductor L1. A first terminal of this LC resonator 23 is connected to an input terminal 27 of Gm transconductor 24, and a second terminal is connected to a reference voltage Vref. Similarly, second LC resonator 25 is produced by connecting in parallel a capacitor C2 and an inductor L2. A first terminal of LC resonator 25 is connected to an output terminal 28 of Gm transconductor 24. A second terminal of LC resonator 25 is connected to the reference voltage Vref.
LC resonators 23 and 25 and Gm transconductor 24 form a loop filter for the SD modulator. In the embodiment in FIG. 1, the transfer function of this loop filter can be described by the following equation:
      H    ⁡          (      s      )        =                    G        m            ⁢              ω        0        2            ⁢              ω        1        2            ⁢              L        2            ⁢              s        2                            (                              s            2                    +                      ω            0            2                          )            ⁢              (                              s            2                    +                      ω            1            2                          )            
This transfer function contains two pairs of complex conjugate poles with the following angular frequencies:
      ω    0    =                    1                                            L              1                        ⁢                          C              1                                          ⁢                          ⁢      and      ⁢                          ⁢              ω        1              =          1                                    L            2                    ⁢                      C            2                              
This transfer function contains two zeros in DC, i.e. at zero frequency.
The performance of the SD modulators obtained with architectures based on a coupling with a transconductor is limited. This is due to the large number of active components which increase the noise level and the non-linearity, and therefore degrade the SNR. Moreover, these components increase the power consumption of the modulator.
With the aim of eliminating this coupling transconductor, it was proposed in U.S. Pat. No. 7,057,541 to produce high-order LC filters by connecting several LC resonators in series.
FIG. 2 represents an example of an SD modulator comprising a loop filter formed by two LC resonators in series. Modulator 4 comprises a processing chain 5 and a current feedback loop 3, not shown in detail. Processing chain 5 receives an analogue signal X(s) at an input 51 and delivers a digital signal Y(z) at an output 52. In this modulator 4, processing chain 5 comprises a loop filter 6 formed by a first LC resonator 61 and a second LC resonator 62. Each LC resonator 61, 62 is formed by connecting in parallel a capacitor C1 or C2, and an inductor L1 or L2, respectively. A first terminal of LC resonator 61 is connected to input 51, upstream of threshold comparator 26, a second terminal of LC resonator 61 is connected to a first terminal of LC resonator 62, and a second terminal of LC resonator 62 is connected to a reference voltage Vref.
In this embodiment, the transfer function of loop filter 6 of modulator 4 can be described by the following equation:
      H    ⁡          (      s      )        =            s      ⁡              (                                            L              1                        ⁢                          C              2                        ⁢                          L              2                        ⁢                          s              2                                +                                    C              1                        ⁢                          L              1                        ⁢                          L              2                        ⁢                          s              2                                +                      L            2                    +                      L            1                          )                            (                                            C              1                        ⁢                          L              1                        ⁢                          s              2                                +          1                )            ⁢              (                                            C              2                        ⁢                          L              2                        ⁢                          s              2                                +          1                )            
This transfer function contains, as in the case of the LC resonators coupled by a Gm transconductor, two pairs of complex conjugate poles having the following angular frequencies:
      ω    0    =                    1                                            L              1                        ⁢                          C              1                                          ⁢                          ⁢      and      ⁢                          ⁢              ω        1              =          1                                    L            2                    ⁢                      C            2                              
This transfer function contains a zero in DC and a pair of complex conjugate zeros at the following angular frequency:
      ω    Z    =                              L          2                +                  L          1                                      L          1                ⁢                              L            2                    ⁡                      (                                          C                1                            +                              C                2                                      )                              
This pair of complex conjugate zeros creates anti-resonance at a frequency close to the resonance frequency of the two LC resonators 61, 62. This anti-resonance frequency in the transfer function of feedback filter 6 makes the following design stages very difficult:                stabilizing the feedback loop of the Sigma-Delta modulator;        designing the noise transfer function so as to maximize the signal-to-noise ratio;        designing the signal transfer function so as to avoid changing the bandwidth of interest and to maximize the attenuation outside the bandwidth.U.S. Pat. No. 7,057,541 does not mention these difficulties and does not offer any technique for overcoming them.        
The purpose of the invention is to overcome the aforementioned drawbacks. Thus, it aims to improve performance in terms of noise and non-linearity of standard architectures using LC resonators coupled via transconductors, while still avoiding the introduction of problems of stabilization of the feedback loop and difficulties of design of the noise and signal transfer functions.
Another purpose of the invention is to propose a low-consumption continuous-time sigma delta radio frequency modulator.
Another purpose of the invention is to propose a continuous-time sigma delta radio frequency modulator having a simple design while offering improved performance in terms of signal-to-noise ratio and linearity.
Finally, another purpose of the present invention is to propose a continuous-time sigma delta radio frequency modulator that is easier to tune.